In the past, there have been proposed an illumination system, which comprises a lighting apparatus and an object detection device comprising a sensor (a motion sensor). Then, the illumination system detects the presence or absence of a detection target object (e.g., a person) in a predetermined detection area via the object detection device, and activates the lighting apparatus during a predetermined time period upon detecting the presence. Thus, the illumination system can activate or deactivate the lighting apparatus automatically, and can prevent a user from forgetting to turn the lighting apparatus off and then can improve convenience and energy conservation. Especially, the illumination system is generally used in locations, such as stairs and passages of establishments, where it is necessary to activate the lighting apparatus to ensure security of a passerby, and where there is no problem if the lighting apparatus is activated only when there is the passerby.
Incidentally, in this kind of the illumination system, in the past, a PIR (Passive Infrared Ray) sensor mainly has been used for the object detection device. The PIR sensor is a passive type sensor, and senses temperature variations caused by a person's motion in a detection area, and outputs a sensor signal. This sensor is relatively cheap and can be used easily.
However, when the above illumination system with the sensor is installed in a location, such as a high ceiling in a factory, a distance from the sensor to the detection target object increases. Therefore, it is difficult to use the PIR sensor having a relatively short sensing distance. So, an active sensor having a relatively long sensing distance is used in such a location. The active sensor itself transmits a detection wave such as an electromagnetic wave, and then receives the detection wave reflected by the detection target object, and thereby detects the presence or absence of the detection target object in the detection area
For example, the active sensor includes a millimeter-wave sensor which transmits a millimeter wave as the detection wave and a distance measuring sensor. The millimeter-wave sensor comprises a Doppler sensor. The Doppler sensor transmits the millimeter wave to the detection area, and then receives the millimeter wave reflected by the detection target object moving in the detection area, and then outputs a sensor signal having a frequency, which is equivalent to a frequency difference between the transmitted millimeter wave and the received millimeter wave. The object detection device comprising the sensor determines the presence or absence of object moving in the detection area, based on the sensor signal outputted from the sensor, and controls to a lighting state of the lighting apparatus (e.g., see Japanese Patent Application Laid-Open No. 2009-168778).
However, if there is a noise source producing a noise of a particular frequency around the object detection device with the active sensor, the noise decreases a signal-to-noise ratio of the object detection device. Therefore, there is a problem that the sensor signal is buried in the noise and the object detection device can not detect the sensor signal, and thus sensitivity of detection decreases.